1. Field of the Invention
The present invention relates generally to an apparatus for detecting a position of an optical pickup and is directed more particularly to an apparatus for detecting a position of an optical pickup for use with an optical reproducing apparatus for a video or PCM (pulse code modulation) audio disc.
2. Description of the Prior Art
In the art, there has been proposed such an optical reproducing apparatus in which the laser beam from a laser light source is converged by an objective lens on the record surface of an optical disc, the reflected laser beam thereon is introduced to the detecting plane of a photo detector and a reproduced signal is derived from the photo detector.
In such the optical reproducing apparatus, there has been disclosed in the U.S. Pat. No. 4,023,033 such a focus servo technique in which the light detecting plane of the photo detector is made of a circular-shape; this light detecting plane is divided equally to four detecting portions of first to fourth quadrants; a semi-cylindrical lens is located in front of the photo detector; a focus state detecting output is provided from the difference between the sum of detected outputs from the detecting portions of the first and third quadrants and the sum of detected outputs from the detecting portions of the second and fourth quadrants; and the position of an objective lens on its optical axis is controlled by the focus state detecting output to thereby carry out the focus servo.
Now, the above focus servo technique will be explained with reference to FIGS. 1 through 6. In FIG. 1, reference numeral 1 generally designates a semi-cylindrical lens, 2 its semi-cylindrical surface, and 3 its rectangular flat plane, respectively. On the plane 3, there is assumed the orthogonal coordinate in which its center point or origin 0 is taken on the center of the plane 3, its X-axis is parallel to the generatrix of the semi-cylindrical surface 2 and passes through the origin 0, the Y-axis is perpendicular to the generatrix or X-axis and passes through the origin 0 and the Z-axis is perpendicular to the plane 3 and passes through the origin 0. Further, a point, which is at the side of the semi-cylindrical surface 3 of the lens 1 and on the Z-axis but apart from the origin 0 by a predetermined distance, is taken as another origin 0', and a rectangular coordinate is assumed with the origin 0' is at its center in which, within the plane perpendicular to the Z-axis, x- and y-axes are considered respectively intersecting the X- and Y-axes by 45.degree. in their positive sides. Then, the circular detecting plane of the photo detector exists in the x and y plane (plane including x- and y-axes) and the detecting plane is divided equally to four portions by the x- and y-axes to thereby divide the photo detector to light detecting portions of first to fourth quadrants I to IV.
Then, a converging beam is incident on the plane 3 of the lens 1 in such a manner that, as shown in FIG. 2, the optical axis of a beam spot 4 formed by the converging beam is coincident with the Z-axis and the spot 4 on the plane 3 becomes a circular shape (FIG. 2 shows the spot 4 in a perspective view). Temporarily, axes a, b, c and d, each corresponding to the radius of the spot 4, are provided in the first to fourth quadrants I to IV at the positions intersecting the X- and Y-axes at 45.degree., respectively.
Now, with reference to FIG. 3, the loci of light rays 5 and 6 on the cross-section of the lens 1 (referred to as X0Z cross-section) including the X-axis, origin 0 and the Z-axis and on the cross-section of the lens 1 (referred to as Y0Z cross-section) including the Y-axis, origin 0 and the Z-axis when the converging beam is incident on the plane 3 of the lens 1 will be explained. Since the Y0Z cross-section is constant in thickness, the ray 6 incident on the Y0Z cross-section advances parallel to the ray before being incident thereon after passing therethrough and then passes through a point P on the Z-axis. While, since the X0Z cross-section forms a convex lens, the ray 5 incident on the X0Z cross-section is refracted to the side of the Z-axis and hence passes through a point P' nearer to the lens 1 than the point P.
It is assumed that the detecting plane DT of the photodetector is positioned at the point 0' (refer to FIG. 1) on the Z-axis between the points P and P' and when a spot 4' of the laser beam irradiated on the detecting plane DT becomes a circle as shown in FIG. 5, the converging beam from the objective lens is focussed on the record surface of the optical disc as a focal point. Thus, the fact that the focal point of the converging beam from the objective lens is formed before or after the record surface of the optical disc is equivalent to the fact that the position of the detecting plane DT of the photo detector is shifted to a point .alpha. before the point 0' or to a point .beta. after the point 0' and hence the spot 4' on the detecting plane DT of the photo detector becomes an ellipse as shown in FIG. 4 or 6. That is, the elliptic spot 4 in case of FIG. 4 has its longer diameter in the direction at 45.degree. relative to the x-axis in the first and third quadrants I and III, while the elliptic spot 4' in case of FIG. 6 has its longer diameter in the direction at 45.degree. relative to the x-axis in the second and fourth quadrants II and IV. In FIGS. 4 to 6, axes a', b', c' and d' correspond to those a, b, c and d of FIG. 2, respectively.
Therefore, if the detecting plane of the photo detector is larger than the spot 4' in the area, based upon the difference between the sum of the detected outputs from the detecting portions of the first and third quadrants I and III and the sum of the detected outputs from the detecting portions of the second and fourth quadrants II and IV, the converging state of the laser beam on the record surface of the optical record medium such as an optical disc by the objective lens can be detected. Accordingly, the focus servo becomes possible by moving the objective lens along its optical axis so as to make the above difference zero.
However, the beam passed through the above semi-cylindrical lens 1 is not kept in similarity of beam distribution, so that when the semi-cylindrical lens is combined with the photo detector, it is difficult to obtain a correct tracking error signal (separated from a focus error signal) by the symmetry of light distribution from the photo detector.